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Oceanic lithosphere carries volatiles, notably water, into the mantle through subduction at convergent plate boundaries. This subducted water exercises control on the production of magma, earthquakes, formation of continental crust and mineral resources. Identifying different potential fluid sources (sediments, crust and mantle lithosphere) and tracing fluids from their release to the surface has proved challenging1. Atlantic subduction zones are a valuable endmember when studying this deep water cycle because hydration in Atlantic lithosphere, produced by slow spreading, is expected to be highly non-uniform2. Here, as part of a multi-disciplinary project in the Lesser Antilles volcanic arc3, we studied boron trace element and isotopic fingerprints of melt inclusions. These reveal that serpentine—that is, hydrated mantle rather than crust or sediments—is a dominant supplier of subducted water to the central arc. This serpentine is most likely to reside in a set of major fracture zones subducted beneath the central arc over approximately the past ten million years. The current dehydration of these fracture zones coincides with the current locations of the highest rates of earthquakes and prominent low shear velocities, whereas the preceding history of dehydration is consistent with the locations of higher volcanic productivity and thicker arc crust. These combined geochemical and geophysical data indicate that the structure and hydration of the subducted plate are directly connected to the evolution of the arc and its associated seismic and volcanic hazards. Serpentine subducted below the Lesser Antilles volcanic arc supplies water to the arc, controlling the location of seismicity, volcanic productivity and thickness of crust.

Pliocene to Quaternary magmatism in the Karacadağ Volcanic Complex in SE Turkey occurred in the foreland region of the Arabia-Eurasia collision and can be divided into two phases. The earlier Karacadağ phase formed a north-south-trending volcanic ridge that erupted three groups of lavas. The same range of mantle sources contributed to the younger Ovabağ phase lavas, which were erupted from monogenetic cones to the east of the Karacadağ fissure. As at several other intraplate localities across the northern Arabian Plate this magmatism represents mixtures of melt from shallow, isotopically enriched mantle and from deeper, more depleted mantle. The deep source is similar to the depleted mantle invoked for other northern Arabian intraplate volcanic fields but at Karacadağ this source contained phlogopite. This source could be located in the shallow convecting mantle or may represent a metasomatic layer in the base of the lithosphere. There is no evidence for a contribution from the Afar mantle plume, as has been proposed elsewhere in northern Arabia. Melting during the Karacadağ and Ovabağ phases could have resulted from a combination of upwelling beneath weak or thinned lithosphere and restricted local extension of that weakened lithosphere as it collided with Eurasia. Tension associated with the collision focused magma of the Karacadağ phase into the elongate shield volcano of Mt. Karacadağ. The northern end of the fissure accommodated more extensive differentiation of magma, with isolated cases of crustal contamination, consistent with greater stress in the lithosphere closest to the collision. Most magma batches of the Karacadağ and Ovabağ phases differentiated by fractional crystallization at ∼5 MPa, near the boundary between the upper and lower crust. Magma batches dominated by melt from garnet lherzolite show evidence for restricted amounts of differentiation at ∼22·5 MPa, which is close to the base of the lithospheric mantle